The invention relates to improvements in measuring instruments and methods wherein an illumination arrangement emits electromagnetic radiation such as visible light into a specimen region. A measuring arrangement is positioned detecting the radiation influenced by the specimen with the radiation converted into a measured signal. More particularly, the invention relates to an improvement in such arrangement wherein a calibration means is provided for lowering the intensity of the radiation proceeding to the specimen during a linearity test of the instrument.
The invention is also directed to a measuring apparatus and method for identifying the properties of a specimen by documentation of the influenced electromagnetic radiation, particularly visible light by the specimen and whereby a controlled lowering of the intensity of the radiation proceeding to the specimen is undertaken for a linearity check.
Instruments and methods for measurements of specimens of the type above referred to are known in a variety of contexts and designs. For example, such known devices enable measurements of and statement outputs concerning the exhaust gas of internal combustion engines and this is often referred to as the opacimetric measuring method in exhaust gas measuring technology. Such measuring instruments and methods according to the opacimeter principle evaluate the attenuation of a radiation stream, particularly a light stream in the visible spectral range, generated by an illumination arrangement. This is detected by a suitable photo element or like arrangement with evaluation electronics on the basis of specimen gas presented in a specimen gas pipe. The attenuation or the extension of the light occurs due to various physical effects including absorption and dispersion.
Other devices and methods in this area of development also operate on the basis of reflection or incident light where the degree of blackening of filter paper is identified by measuring the reflectivity and is employed for output statements about the particle charging of exhaust gas or the like.
A halogen lamp having emission of light over a broad part of the visible spectral range is preferably currently utilized as an illumination source in cases which employ visible light. Other illumination sources, such as light emitting diodes having intensity maximums in the range of green light are also additionally employed.
In the evaluation of the measurement, in one method the measured signal level detected in the photo element of the measuring arrangement is proportional to the incident light stream, that is corresponding to the luminous intensity. This means that the measuring instrument basically works linearly.
In the practical realization, however, linearity errors in the measuring arrangement, particularly the saturation phenomena of the photo element, and errors in the evaluation electronics can be identified in addition to the shift to the zero point when the illumination arrangement is shut off. This is particularly due to dark currents, the entry of extraneous light, and offset errors of the evaluation electronics. This is in addition to a change in the sensitivity of the measuring arrangement, particularly triggered by aging of the illumination arrangement and of the photo element or the corresponding measuring arrangement, contamination of the beam path, and drift phenomena of the evaluation electronics.
In addition to containing the possibility of calibrating the zero point when the illumination arrangement is shut off, and of calibrating the maximum value with a clean specimen, standard embodiments of measuring instruments of the type described also contain a calibration means. Such calibration means is used in order to check the linearity of the measuring instrument in the individual measurement ranges with simple means, usually an individual measuring point in the middle of the measuring range.
The state of the art for the implementation of this simple linearity check is a filter disk with defined attenuation that can be inserted into the illumination path or in some instances, an insertable mechanical diaphragm with a defined aperture. This filter disk, or diaphragm, can be manually introduced or can be introduced with the assistance of an adjustment element such as a lifter magnet or an electric motor for automatic operation of the measuring instrument. In both instances, an optimally exactly defined part of the flowing illumination stream is occluded and the measured result identified by the measuring instrument as compared to the defined value.
In addition to the considerable mechanical outlay required for automated implementation, the disadvantage of linearity checks of a mechanism with diaphragms is that the geometric relationships enter into the measured result. For instance, the propagation of light through the specimen gas tube of an opacimeter, usually does not ensue in the form of parallel light rays. The outlay is therefore kept within justifiable limits. In particular, costly lenses or mirror systems are avoided. Changes in the light stream as particularly occur due to tolerances in the region of a lamp holder of the measurement head of the specimen gas pipe and of the diaphragm can, therefore, exhibit direct influence on the measured result given the linearity test with a mechanical diaphragm. The measurement error effected by said influences lies in the percentage range of known standard measuring instruments and method of the type referred to and thus lies one through several orders of magnitude above the typical linearity errors themselves.
Filter disks having defined attenuation of the illumination, that do not have the disadvantage of direct dependency on the geometrical arrangement, and that are therefore utilized for calibrations under laboratory conditions can only be utilized for an automated linearity check with considerable outlay. In particular, a falsification of the degree of attenuation due to the potential contamination under more difficult environment situations, as apply particularly to a factory measuring instrument, must be prevented.
It is accordingly an object of the present invention to provide an improved measuring instrument and method of the general type referred to such that the disadvantages of known instruments and methods do not occur such that exact and reliable linearity checks can be implemented.
A further object of the invention is to provide a measuring instrument so that simple and reliable means for linearity checks can be provided without expensive equipment construction.
A further object of the invention is to provide a measuring instrument of the type referred to wherein quick and reliable linearity checks can be conducted.